Highly transparent to truly black electrochromic devices based on an ambipolar system of polyamides and viologen

Stray light that makes it hard to spot objects in see-through displays can be reduced by 95% with a new colour-changing polymer. The approach, from Guey-Sheng Liou of the National Taiwan University and co-workers, involves solution casting a blend of electrochromic materials to achieve a ‘transparent-to-black’ transformation for contrast-improving shutters. The primary component is an anodic electrochromic film made from two triarylamine-based polymers with reversible electrochemical properties. When a gel electrolyte containing cathodic viologen molecules is sandwiched between two ITO-coated glass electrodes with the polymer film, the device could merge the colors of red, green or blue and turned from transparent to black with just mild voltage stimulation, thanks to built-in charge trapping effects. These colours spanned most of the visible-light spectrum and could be subtracted to generate the final target colour of black.